The thermal power system tends to raise the steam temperature of a steam turbine in order to make the generating efficiency much higher. As a result, high temperature characteristics demanded for the cast steel material to be used for the steam turbine also become much stricter.
There have been proposed many heat resistant cast steels as cast steel materials to be used for steam turbines.
It is necessary to improve a long creep rupture life of a heat resistant cast steel material which is used for the steam turbine in order to contribute to further improvement of the generating efficiency. In a case where a large cast material such as a turbine casing or a high temperature valve casing of the steam turbine is configured, it is especially required that the heat resistant cast steel material has good quality. Specifically, the heat resistant cast steel material is required that the molten metal has excellent fluidity when casting, cast defects such as gas holes, shrinkage cavities, and hot tears are not many, and component segregation at each portion of the material is not large. If a cast defect occurs, that portion is repaired by welding, so that the heat resistant cast steel material used for the steam turbine is also required to have excellent weldability.
Factors which influence the quality of cast parts include a casting method, chemical component elements of the material configuring the cast parts, and the like. Therefore, it is necessary to select optimum chemical component elements of material in conformity with the cast parts to be produced.
In addition, the heat resistant cast steel material used for the steam turbine is required to have characteristics excellent in the creep rupture life and also in creep ductility and toughness from a viewpoint of the fracture prevention during the operation of the steam turbine. And, when the heat resistant cast steel is subjected to a long aging process at a high temperature or long creep degradation, creep rupture ductility and toughness might be reduced. When such reduction occurs in a large structural component such as a turbine casing or a high temperature valve, an operational risk increases.
Therefore, it is important to provide a product having high reliability for a long term for the heat resistant cast steel material to be used for the steam turbine considering the reduction in strength, ductility and toughness due to aging deterioration of the material.
It is very difficult to achieve all of the above-described improvements of a long creep rupture life, creep rupture ductility and toughness, and also suppression of aging deterioration after a high temperature long-term operation.